If Phred throws a rock straight up, and Phran throws a rock straight up with twice the velocity of Pread's rock, how will the maximum height of Phred's rock compare to Phran's?

The maximum height of Phran stone is four time that of Phred stone

Explanation:

From the question we are told that

   The initial velocity of Phred stone is  u

    The initial velocity of  Phran stone is  2u

Generally from kinematic equations

Here the negative sign shows that the direction of the stone is against gravity

At the maximum height v which is the final velocity of the stone is zero  

So

So for Phred stone the height attained is  

=>  

For  Phran stone the height attained is  

=>  

=>    

=>    

Hence the maximum height of Phran stone is four time that of Phred stone